Seismic Performance of Prefabricated Steel Pipe Recycled Concrete Frame Joints with Different Configurations

Literature Overview

This study by Bian Jinliang, Cao Wanlin, Zhang Zongmin, and Ye Taoping from Beijing University of Technology and Tianjin Chengjian University investigates the seismic performance of different joint configurations for prefabricated steel pipe recycled concrete frames. Published in Building Structure (Volume 51, Issue 5, 2021, pages 67–74), the research was supported by the National Key R&D Program of China (13th Five-Year Plan, Grant 2018YFD1100903). The study addresses the growing demand for sustainable construction through the use of recycled concrete aggregates combined with prefabricated construction methods, while ensuring adequate seismic performance.

Core Technical Content and Methodology

Research Background

The integration of recycled concrete (concrete incorporating recycled coarse aggregates from demolished concrete) with prefabricated steel pipe frame construction represents a convergence of sustainability and construction efficiency. However, the seismic performance of joints in such systems is critical and was previously poorly understood. The study proposes a novel double L-shaped joint with stiffening ribs and compares its performance against several alternative joint configurations.

Joint Configurations Investigated

Four distinct joint configurations were tested:

Double L-shaped joint with stiffening ribs (proposed joint): Composed of double L-shaped stiffened components welded to the steel pipe column, the joint region steel pipe, and beam segment regions on the L-shaped components.
Double L-shaped joint without stiffening ribs: Same basic geometry as the proposed joint but without the triangular stiffening ribs.
Straight plate connection joint: Uses flat steel plates for beam-column connection.

Two-plate configuration
Three-plate configuration

Welded joint: Direct welding of beam to column (conventional approach).

Experimental Program

Low-cycle reversed loading tests were conducted on all four joint configurations to simulate seismic loading conditions. The tests evaluated:

Ductile failure characteristics
Load-bearing capacity
Hysteretic behavior
Ductility
Stiffness degradation
Energy dissipation capacity

Test Results Summary

Joint Configuration	Load Capacity	Ductility	Stiffness	Energy Dissipation	Failure Mode
Double L with stiffening ribs	Highest	High	High	Highest	Beam plastic hinge
Double L without stiffening ribs	Moderate	Moderate	Moderate	Moderate	Beam-column interface
Two-plate connection	Good	Good	Good	Good	Beam plastic hinge
Three-plate connection	Moderate	Moderate	Moderate	Moderate	Plate yielding
Welded joint	Low	Low	Low	Low	Weld fracture

Key Findings

Significant influence of joint configuration: The seismic performance varies dramatically depending on the joint design, confirming that joint design is the governing factor in prefabricated frame seismic behavior.
Welded joints perform poorly: Direct welded joints exhibited the lowest load capacity and ductility, with brittle failure modes that are unacceptable for seismic design.
Two-plate configuration superior to three-plate: Among straight plate connections, the two-plate configuration outperformed the three-plate configuration in seismic performance.
Stiffening ribs significantly enhance performance: The addition of triangular stiffening ribs to the double L-shaped joint substantially improved load capacity, stiffness, and energy dissipation.
Simple construction with high performance: The proposed double L-shaped joint with stiffening ribs offers a balance of construction simplicity and excellent seismic performance.

Process and Standards Analysis

Connection to Seismic Design Codes

The findings have implications for the application of:

GB 50011: Code for Seismic Design of Buildings
GB 51249: Technical Standard for Prefabricated Concrete Building
JGJ/T 1-2019: Technical Specification for Prefabricated Concrete Components
GB 51232: Standard for Design of Concrete Structures (recycled concrete provisions)

The study's emphasis on ductile failure modes and energy dissipation aligns with the capacity design philosophy of modern seismic codes, which require that plastic deformation occur in designated ductile elements rather than in brittle connections.

Recycled Concrete Considerations

The use of recycled concrete aggregates introduces material variability that must be considered in joint design:

Higher water absorption of recycled aggregates
Potentially lower concrete strength variability
Possible interfacial transition zone (ITZ) weakness between recycled aggregates and new cement paste
Potential for higher permeability

These material characteristics influence the concrete's contribution to joint behavior, particularly in the confinement and shear transfer mechanisms.

Welding Quality Requirements

The welded components (L-shaped plates to steel pipe columns) must meet stringent welding quality requirements:

Full penetration welds for primary load-bearing connections
Visual inspection (VT) for 100% of welds
Ultrasonic testing (UT) for critical welds
Weld procedure qualification per GB/T 9445 or ISO 9606

FMEA Analysis of Joint Configurations

Failure Mode	Configuration Susceptibility	Prevention Measure
Weld fracture	Welded joint (high risk)	Use mechanical connections instead
Bolt shear failure	Plate connections (moderate)	Proper bolt sizing and layout
Concrete crushing	All configurations (low-moderate)	Adequate confinement reinforcement
Slip at interface	Double L without ribs (moderate)	Add stiffening ribs
Beam-column rotation capacity	All (depends on design)	Ensure plastic hinge in beam

Integration with Engineering Practice

Construction Methodology

The prefabricated steel pipe recycled concrete frame system involves:

Off-site manufacturing: Steel pipe columns, beam segments, and joint components are fabricated in controlled factory conditions
Recycled concrete production: Concrete with recycled coarse aggregates is batched and placed in controlled conditions
Site assembly: Components are lifted and connected using the designed joint configurations
On-site concrete placement: Recycled concrete is placed in beam and slab regions

Quality Control Measures

Critical quality control points for the proposed joint system include:

Factory welding inspection: All welds on L-shaped components must be inspected before shipment
Surface preparation: Contact surfaces must be properly prepared for reliable bearing
Bolt torque verification: High-strength bolts must be torqued to specified values
Concrete quality: Recycled concrete must meet specified strength and durability requirements
As-built survey: Joint alignment and fit-up must be verified during assembly

Design Recommendations

Based on the test results, the following design recommendations emerge:

Preferred configuration: Double L-shaped joint with stiffening ribs for seismic zones
Stiffening rib height: Increase rib height to improve seismic performance (diminishing returns beyond optimal height)
Avoid welded joints: Direct welded connections are unacceptable for seismic design of prefabricated frames
Two-plate over three-plate: When plate connections are used, prefer two-plate configuration
Capacity design: Ensure the joint can sustain the full plastic moment capacity of the connected beam

Key Questions and Reflections

Recycled Concrete Long-Term Behavior

While the study demonstrates acceptable seismic performance with recycled concrete, long-term durability concerns remain. The interfacial transition zone between recycled aggregates and cement paste may degrade over time due to carbonation, chloride ingress, or freeze-thaw cycles. This could potentially affect the concrete's confinement contribution to joint performance over the service life of the structure.

Scale Effects

The test specimens were likely at full scale or near full scale, but the behavior of large-scale prefabricated frames with multiple stories and spans may differ from isolated joint tests. Frame-level effects, including P-delta effects, story drift demands, and interaction between multiple joints, require further investigation through full-scale or large-scale frame tests.

Economic Considerations

The additional cost of stiffening ribs must be weighed against the seismic performance benefits. In high seismic zones, the improved performance may justify the additional material and fabrication cost. In low seismic zones, simpler configurations may be acceptable, potentially reducing costs.

Study Insights and Implications

This research makes a significant contribution to the field of sustainable structural engineering by demonstrating that recycled concrete can be effectively used in prefabricated steel pipe frame systems without compromising seismic performance, provided that appropriate joint configurations are employed. The proposed double L-shaped joint with stiffening ribs represents a practical solution that balances construction simplicity with structural performance. The finding that welded joints perform poorly in this context is particularly important, as it challenges the conventional assumption that welding always provides the strongest connection. For prefabricated construction, mechanical connections that allow for field adjustment and inspection are clearly superior. The study's systematic comparison of multiple joint configurations provides a valuable decision-making framework for engineers selecting joint types for different seismic performance requirements. Future research should extend these findings to include long-term durability studies, cyclic loading with larger displacements, and frame-level tests to validate the isolated joint test results in a more realistic structural context.